1. Field of the Invention
This invention relates to a method of dividing a circuit pattern to form a plurality of complementary patterns, a method of manufacturing a stencil mask composed of a combination of a plurality of complementary stencil masks corresponding to a plurality of partitioned complementary patterns, a stencil mask obtained by such a manufacturing method, and a method of exposure using such a stencil mask.
2. Description of the Related Art
In recent years, concomitant with a trend to further enhance the fineness of a semiconductor integrated circuit, the research and development are now extensively conducted with regard to electron beam lithography, ion beam lithography, etc. as manufacturing techniques of the semiconductor integrated circuit. The stencil mask employed in these lithography techniques is constituted of an Si self-supporting membrane having a thickness of not more than 2 μm and a transfer through-hole pattern formed therein.
Among these lithography techniques, there has been developed, as a method suitable for coping with the miniaturization of semiconductor devices, an electron beam exposure method such as a method called a cell projection exposure method or a block exposure method, where an electron beam is employed, or a method called EPL (Electron Projection Lithography), which is capable of realizing a higher throughput.
In the EPL method, a circuit pattern is transferred onto a photosensitive layer-coated substrate 410 by making use of a stencil mask 400 as shown in FIG. 1. In this stencil mask, a circuit pattern of fine through-holes 371, i.e. a stencil (openings) is formed in a self-supporting membrane 331a of predetermined thickness. An electron beam is passed through the fine through-holes 371, irradiating onto a sensitive substrate 410 in a form of the circuit pattern, thus performing an electron beam projection exposure.
As for the thickness of the self-supporting membrane 331a, it is more preferable that the thickness be as large as possible in order to enable the electron beam to disperse and to enhance the rigidity of the self-supporting membrane. On the other hand, in view of problems such as workability of the transfer through-hole pattern and the gravity deflection of the self-supporting membrane, there are limitations with regard to the thickness of the self-supporting membrane. For example, in the case of a self-supporting material having a modulus of longitudinal elasticity ranging from about 100 to 200 GPa, such as Si, there is known a self-supporting membrane having a thickness of about 0.4 to 3 μm though specific thickness of the self-supporting membrane may vary depending on the magnitude of the acceleration voltage of the electron beam to be irradiated.
However, in the case where a fine through-hole having a width of not more than several micrometers and a length larger than the width is to be formed in such a self-supporting membrane having a thickness of several micrometers as described above, if a beam-like member such as a cantilevered beam member where three sides thereof are surrounded by a fine through-hole or a both end-supported beam member which is located between a pair of fine through-holes, is formed, there will be raised not only the problem of deformation due to the gravity deflection of the beam-like member, but also the problem of generating the phenomenon that, due to the deformation of the beam-like member, the beam-like members are permitted to be in contact with each other or any one of the beam-like members is permitted to be in contact with a neighboring circuit pattern. For the purpose of overcoming these problems, it is conceivable to partition the circuit pattern formed in the mask into a plurality of complementary patterns so as to obtain a complementary stencil mask having a plurality of complementary transfer through-hole patterns corresponding to these complementary patterns.
With respect to the method of dividing a circuit pattern into a plurality of complementary patterns, there is known a method wherein an upper limit of the ratio between the length L and the width W of a beam-like member pattern (L/W), i.e. an upper limit of aspect ratio is set to a predetermined value at first, and then the circuit pattern is partitioned so as to make the aspect ratio lower than the value. Next, a dividing method of the circuit pattern as well as a method of manufacturing a stencil mask composed of a combination of complementary stencil masks each having a complementary transfer through-hole pattern corresponding to each of the complementary patterns partitioned as described above will be explained.
First of all, in a step of designing a mask pattern, a cantilevered beam member pattern 311 where three sides thereof are surrounded by a through-hole pattern 312 in a transfer pattern 310, or a both end-supported beam member pattern 321 which is interposed between fine through-hole patterns 322 in a transfer pattern 320 as shown in FIGS. 2A and 2B is detected. These transfer patterns 310 and 320 are then partitioned so as to enable the aspect ratio (La/Wa) of the cantilevered beam member pattern 311 as well as the aspect ratio (Lb/Wb) of the both end-supported beam member pattern 321 to have predetermined set values.
Namely, as shown in FIG. 3, the transfer pattern 310 is partitioned into a first complementary transfer pattern 310a consisting of a first complementary through-hole pattern 312a and a first complementary beam-like member pattern 311a, and a second complementary transfer pattern 310b consisting of a second complementary through-hole pattern 312b and a second complementary beam-like member pattern 311b. Further, as shown in FIG. 4, the transfer pattern 320 is partitioned into a first complementary transfer pattern 320a consisting of a first complementary through-hole pattern 322a and a first complementary beam-like member pattern 321a, and a second complementary transfer pattern 320b consisting of a second complementary through-hole pattern 322b and a second complementary beam-like member pattern 321b. 
A method of manufacturing a complementary stencil mask having patterned configurations corresponding to the partitioned complementary transfer patterns 310a and 310b, respectively, will next be explained with reference to FIGS. 5A to 5F.
First of all, as shown in FIG. 5A, an SOI substrate 330 composed of an Si active layer 331 having a thickness of 2 μm, an intermediate oxide film layer 332 having a thickness of 1 μm, and a Si supporting layer 333 having a thickness of 525 μm is prepared.
Then, a photoresist is coated on the surface of the Si supporting layer 333 to form a photoresist film which is worked by means of photolithography to form a resist pattern 341 as shown in FIG. 5B. Thereafter, as shown in FIG. 5C, by making use of this resist pattern 341 as a mask, the Si supporting layer 333 is etched away by means of a known dry etching method until the intermediate oxide film layer 332 is exposed, thereby forming an opening 351 of the supporting layer.
Thereafter, the resist pattern 341 deposited on the Si supporting layer 333 is removed by means of ashing treatment, etc. and the intermediate oxide film layer 332 exposed through the opening 351 of the supporting layer is removed by making use of a 3 wt % aqueous solution of HF, etc. to thereby form an opening 351 of the supporting layer. Further, the resultant substrate is subjected to a cleaning treatment by way of the conventional RCA cleaning, etc. to thereby obtain a blank 330a for stencil mask (hereinafter referred to as a stencil mask blank 330a) as shown in FIG. 5D.
Subsequently, an electron-beam-sensitive resist is coated on the surface of the Si active layer 331 of the stencil mask blank 330a and the resultant electron-beam-sensitive resist layer is subjected to an electron beam exposure by making use of the pattern data of the aforementioned complementary transfer patterns 310a and 310b. Then, the electron-beam-sensitive resist layer is further subjected to a development treatment to obtain a complementary resist pattern 361 as shown in FIG. 5E.
Thereafter, by making use of the complementary resist pattern 361 as an etching mask, the Si active layer 331 is etched away by means of the conventional plasma etching using an etching gas such as SF6, fluorocarbon, etc. Then, the complementary resist pattern 361 is removed and the resultant substrate is subjected to a cleaning treatment to obtain two complementary stencil masks 400 each having a self-supporting membrane 331a which is constituted of a self-supporting membrane consisting of the Si active layer 331 and having fine through-holes 371 formed therein as shown in FIG. 5F.
Further, by way of the same procedures as described above, another two complementary stencil masks, each having a prescribed pattern, can be obtained in accordance with the aforementioned complementary transfer patterns 320a and 320b. 
FIG. 6 shows an example of a transfer pattern which is formed through a combination of complementary stencil masks corresponding with the complementary transfer patterns which are partitioned according to the aforementioned conventional method.
In the case of the conventional complementary stencil mask, since the preset value of the aspect ratio of the partitioned beam-like member patterns 311a and 311b (Lc/Wc, Ld/Wd) is made constant, the beam-like member patterns 311a and 311b are partitioned according to the predetermined aspect ratio irrespective of the values the widths Wc and Wd of the beam-like member patterns 311a and 311b would take. However, depending on the width of the support portion or on the configuration of the beam-like member, it is possible to form the beam-like member patterns 311a and 311b having a much larger aspect ratio. Namely, depending on the width of the support portion or on the configuration of the beam-like member, the dividing of the beam-like member patterns has been conventionally performed to an unnecessary extent in number of partitions.
The increase in number of partitions would raise not only a problem that it takes much time in dividing a transfer pattern into complementary patterns in the step of designing the mask pattern of a beam-like member, but also a problem that the quantity of data on the complementary patterns is caused to increase, thereby prolonging the time for handling the data.
Additionally, when the number of the fine through-holes having a small opening is increased due to unnecessary dividing, the number of defective mask is caused to increase due to the fact that any foreign matter that may be present in the fine through-hole cannot be removed in the cleaning step of the mask, thereby raising a problem that the yield will be deteriorated in the manufacture of a mask.
Further, since the complementary patterns thus partitioned are designed to be connected with each other so as to obtain a prescribed pattern in the exposure step for transferring the pattern to the photosensitive layer formed on a semiconductor substrate, the superfluous dividing would lead to an increase in number of connected portions, thus raising the problem that the risk of generating defective masks due to defective connection of the complementary patterns would be increased.
Further, as an alternative method, there has been conventionally proposed a method wherein the width as well as the length of the through pattern of circuit pattern are detected at first and then compared, to select the longer width and length and, based on this selection, the transfer pattern is partitioned in a manner such that the selected one is not larger than a preset value.
This conventional method however is accompanied with a problem that since the mechanical strength of the beam-like member of the stencil mask to be created by a transfer pattern is not taken into consideration, where the aspect ratio of the beam-like member and the width of the support portion are selected to satisfy prescribed values, the beam-like members may adhere to each other or any one of the beam-like members may adhere to a neighboring circuit pattern, depending on the manufacturing process of the mask.
Therefore, one of the objects of the present invention is to provide a method of dividing a circuit pattern, which makes it possible, on the occasion of dividing a circuit pattern into complementary patterns to perform an efficient dividing of a circuit pattern in a manner to avoid contact between the beam-like members of high aspect ratio or between any one of these beam-like members and a neighboring circuit pattern in the manufacturing process of a stencil mask, thereby making it possible to manufacture a stencil mask with a high yield.
Another object of the present invention is to provide a method of manufacturing a stencil mask which is constituted of a combination of a plurality of complementary stencil masks corresponding to a plurality of complementary patterns which have been partitioned as described above.
A further object of the present invention is to provide a stencil mask which is manufactured according to the aforementioned method.
Another object of the present invention is to provide a method of exposure using the aforementioned stencil mask.